Process for manufacturing alloy steel wires having low relaxation characteristics

ABSTRACT

A PROCESS FOR MANUFACTURING LOW-ALLOY STEEL WIRES HAVING LOW RELAXATION CHARACTERISTICS AT ROOM AND ELEVATED TEMPERATURES CHAACTERIZED BY HEATING A STARTING WIRE MATERIAL MADE OF LOW ALLOY STEEL CONTAINING, AS THE ALLOYING EYEMENT(S), EITHER SLICON ALONE OR A COMBINATION OF SILICON WITH AT LEAST ONE OTHER ELEMENT SUCH AS CHROMIUM, TO A TEMPERATURE ABOVE ITS A3 TRANSFORMATION POINT; EITHER COOLING THE HEATED WIRE MATERIAL AT A SPECIFIC COOLING RATE TO A TEMPERATURE JUST ABOVE ITS MS POINT BY MEANS OF NATURAL OR FORCED AIR COOLED AND REHEATING THE COOLED WIRE MATERIAL, WHEN THE MATERIAL IS SUCH THAT THE SPECIFIC COOLING RATE CAN BE EQUAL TO OR HIGHER THAN THE LOWER CRITICAL COOLING RATE OF THE MATERIAL, OR COOLING THE HEATED WIRE MATERIAL BY PASSING IT THROUGH A BATH OF MOLTEN LEAD OR SALT AT A TEMPERATURE OF 450-650*C. WHEN THE MATERIAL IS SUCH THAT ITS AIR COOLING RATE IA LOWER THAN ITS LOWER CRITICAL COOLING RATE, IN ORDER TO OBTAIN A WIRE MATERIAL HAVING A HIGHLY COLD-WORKABLE SORBITIC STRUCTURE AND HIGH TENSILE STENGTH; AND DRAW WORKING THE THUSOBTAINED WIRE MATERIAL AN THEN ARTIFICIALLY AGING THE DRAW WORKED WIRE MATERIAL TO MANUFACTURE THE DESIRED LOW-ALLOY STEEL WIRES.   D R A W I N G

March 7, 1972 KAZUO OKAMOTO EI'AL 3,647,571

PROCESS FOR MANUFACTURING ALLOY STEEL WIRES HAVING LOW RELAXATION CHARACTERISTICS Filed July 14, 1969 5 Sheets-Sheet 1 A FIG.

RELAXATION (9%) O 50 I00 I50 200 TEMPERATURE OF EXPOSURE (C) |5o- A= c STEEL B= Si STEEL I l l STRESS-RELIEVING TEMPERATURE (C) March 7, 1972 KAZUQ OKAMQTO ETAL 3,647,571

PROCESS FOR MANUFACTURING ALLOY STEEL WIRES HAVING LOW RELAXATION CHARACTERISTICS Filed July 14, 1969 5 Sheets-Sheet 2 FIG. 2

I00- --A-- -A 2 12 a aloon: 30 f 3 f v 2 V c STEEL Si STEEL A-----ACr-Si STEEL AS AS 200 300 400 500 DRAWN ROLLED .STRESS RELIEVING TEMPERATURE F'IG.3

C' STEEL Si STEEL 7 A 6 -----A Cr-Si STEEL 25 a o g 4 g 3 i 5 A k m 2 I m u/ I l I I l l I L I I I l 0 AS AS 200 300 400 500 DRAWN ROLLED STRESS REELIEVING TEMPERATURE (96) March 7, 1972 KAZUQ OKAMOTO ETAL 3,647,571

PROCESS FOR MANUFACTURING ALLOY STEEL WIRES HAVING LOW RELAXATION CHARACTERISTICS I 5 Sheets-Sheet 5 Filed July 14, 1969 FIG-.4A

STEEL STEEL A--A Cr-Si STEEL I00 TEMPERATURE OF EXPOSURE (c) $3 mmmmhw hummmo awm TEMPERATURE OF EXPOSURE (C) C n A gm m A .m F m E I wm I 3 s M w w W X W F 0o LLL W EEE R s EEE U TTT T \A $88 A bl R kl AW M m m T A g I w- TI 0 R 0 m w 6 w 38 mwumhm .mm. .EO $00 March 7, 1972 KAZUQ OKAMOTO EI'AL 3,647,571

PROCESS FOR MANUFACTURING ALLOY STEEL WIRES HAVING LOW RELAXATION CHARACTERISTICS Filed July 14, 1969 5 Sheets-Sheet 5 -FIG.7

*2: 20 9O =5: 1/ 5" so I (D. I m O: I?

a C STEEL -U. 60 Si STEEL Cr-Si STEEL l I l l I l l l 0 l 2 3 4 5 6 7 8 9 l0 RELAXATION FIG.8' TIME-(HR) .l I I0 I00 I000 T\ Q:- A 7 a2 tr i* A v u\ E:I ---a A W. LH 'X'FM 3 CONTINUE Q CONTINUE 2Q C STEEL .....L MY.H 3 |:|-u Si STEEL A---A Cr-Si STEEL United States Patent US. Cl. 14812.3 16 Claims ABSTRACT OF THE DISCLOSURE A process for manu'facniring low-alloy steel wires having low relaxation characteristics at room and elevated temperatures characterized by heating a starting wire material made of low alloy steel containing, as the alloying eyement(s), either silicon alone or a combination of silicon with at least one other element such as chromium, to a temperature above its A transformation point; either cooling the heated wire material at a specific cooling rate to a temperature just above its Ms point by means of natural or forced air cooling and reheating the cooled wire material, when the material is such that the specific cooling rate can be equal to or higher than the lower critical cooling rate of the material, or cooling the heated wire material by passing it through a bath of molten lead or salt at a temperature of 450-650 C. when the material is such that its air cooling rate is lower than its lower critical cooling rate, in order to obtain a wire material having a highly cold-workable sorbitic structure and high tensile strength; and draw working the thusobtained wire material and then artificially aging the draw worked wire material to manufacture the desired low-alloy steel wires.

This invention relates to a process for preparing high strength steel wire having excellent tensile strength at room and elevated temperatures, and low relaxation or low creep characteristics, which includes cold drawing and then artificially aging (aging at elevated temperatures) a wire material made of low-alloy steel of ordinary type. The high strength steel wire according to this invention can be used as that to which is continuously applied such stress as is slightly lower than its yield strength during a long period of several ten years, as seen from those used in prestressed concrete structures, cables for suspension bridges and the like; or it is used as a tensile steel wire, such as prestressed concrete steel wire, for introducing compressive stress into a structure wherein it is used, such as a nuclear reactors primary container made of prestressed concrete, the structure being usually heated to a temperature above room temperature.

Steel wire which has conventionally been used in prestressed concrete structures, is prepared by cold drawing wire material of high carbon steel comprising 0.65-0.95 by weight of carbon, 0.304190% by weight of manganese, up to 0. 35% by weight of silicon and the balance consisting of iron and incidental impurities and then heat treating the resulting cold drawn wire to relieve it of its stress or twisting at least two of the resulting drawn wires to form a strand which is subsequently heat treated to relieve it of its stress. This conventional carbon wire has a high relaxation value, though not intolerably high, and has a disadvantage in that it will sharply increase in relaxation value when placed in an atmosphere heated "ice above room temperature. Another conventional steel wire for prestressed concrete, which has been subjected to heat treatment, that is, quench hardening and tempering to be provided with high strength, has a disadvantage in that it is especially liable to suffer stress corrosion when stressed in a corrosive atmosphere.

In contrast to these prior art processes, the present invention is directed to a process for producing an alloy steel wire having low relaxation property at room and elevated temperatures from a low-alloy steel (silicon or silicon-chromium steel) wire material comprising 0.50- 0.95% by weight of carbon, 0.30-1.00% by weight of manganese, (MO-2.20% by weight of silicon, at least one selected from the group consisting of 0.21.2% of chromium, up to 0.5% of molybdenum, up to 0.5% of titanium, up to 0.5% of niobium and up to 0.5% of vanadium, and the balance consisting of iron and incidental impurities, which process comprises, while continuously passing the starting wire material through a heat treating installation, (1) heating the starting wire material to a temperature which is 50-150 C., preferably 50-100 C., above its A transformation point, (2) cooling the heated wire material at a suitable cooling rate (velocity) to a temperature just above its Ms point, the cooling rate being determined depending on the composition and diameter or thickness of the starting wire material in the form of rod, bar or the like, and the cooling being effected by means of forced air cooling or patenting as in a high carbon steel where the starting material is difficult to cool to the desired temperature by means of natural air cooling because of its larger diameter or thickness, (3) reheating the cooled material to give it a sorbitic structure and high final tensile strength despite the fact that it is a low-alloy steel wire material, and then (4) cold drawing the heattreated material after pickled, neutralized, washed with water and coated with an antifriction agent, to give it work hardness thereby producing the desired alloy steel wire hardness low relaxation property at room and elevated temperatures. The term air cooling used herein is intended to mean natural or forced air cooling.

In the cooling step of the above-mentioned heat treatments, the cooling of the wire material at a cooling rate higher than its lower critical cooling rate may usually and industrially be effected by natural or forced air cooling. However, it may be difiicult or rather impossible to effect the cooling of the wire material at a cooling rate higher than its lower critical cooling rate by means of the air cooling, depending mainly upon the composition and diameter or thickness of the 'wire material, If the wire material is difficult or impossible to cool at such a cooling rate as above because of its greater diameter or thickness and/or its content of the alloying elements, it should 'be passed through a lead or salt bath heated to a predetermined temperature just above its Ms point, instead of air cooling, so that it may conveniently be cooled at a cooling rate higher than its lower critical cooling rate.

The wire material thus cooled to the temperature just above its Ms point, is then reheated to a temperature lower than is A transformation point by approximately C. or less by passing it through an ordinary heating furnace, lead bath, salt bath or the like, followed by allowing it to be cooled in the air to room temperature.

If the wire material is made of silicon steel and has a diameter of not smaller than approximately '6 mm. or it is made of chromium-silicon steel and has a diameter of not smaller than about 9.5 mm., it will usually be difficult or impossible to cool at a cooling rate higher than its lower critical cooling rate by the usual cooling means such as air cooling and its temperature will also be extremely diflicult to control. Such a wire material should thus be heat treated in the same manner as the patenting of a high carbon steel wire material. More particularly, if it is made of silicon steel, it should be heated to a temperature above its A transformation point and then successively dipped in a bath such as a lead bath at 450- 650 C. for not less than 20 seconds or subjected to natural or forced air cooling or the like in such a manner that it may be cooled at a cooling rate of not higher than 3.0 C./sec.; while, if it is made of chromium-silicon steel, it should be heated to a temperature above its A transformation point and then successively dipped in a bath such as a lead bath at 45'0-650 C. for not less than 50 seconds or subjected to natural or forced cooling or the like so as to be cooled at a cooling rate of not higher than C./sec.

It is especially advisable for obtaining good results that a silicon steel wire material of approximately '6 mm. in diameter and a chromium-silicon wire material of nearly 9.5 mm. in each according to this invention, should preliminarily be tested to determine which type of heat treatment according to this invention they should be subjected to by using their samples in the preliminary test prior to subjecting them to the heat treatment.

The conventional low-alloy steel wire is low in strength and is metallographically in spheroidized or coarse-pearlitic structure which is inferior in cold workability, since it is prepared by annealing a wire waterial therefor wound up in a coil in a pot furnace or the like. The conventional wire has also a remarkable drawback in that it is liable to suffer stress corrosion, acid brittleness and hydrogen brittleness during electroplating when in use, since it is generally prepared by oil tempering a wire material therefor and then quench hardening the oiltempered wire material.

According to this invention, a silicon or silicon-chromium steel wire material is heat treated to make it sorbitic in structure, the sorbitic structure being superior in cold workability, thereby decreasing the hazard of its acid brittleness during pickling and its hydrogen brittleness during electroplating, and the subsequent cold drawing of the wire material will further lessen this hazard.

In the practice of this invention, a starting wire material is cold drawn through several dies in series using the known technique until at least 40% of reduction of area is reached at the end of the drawing. The drawn wire material, which is hardened by the drawing work, is then heat treated at a temperature of 250-550 C., preferably 440 C. or higher, to relieve it of its stress and at the same time to subject it to artificial aging, thereby improving it in tensile strength and relaxation property at room and elevated temperature.

It is an object of this invention to provide a process for manufacturing alloy steel wires having a low relaxation or low creep characteristic at room and elevated temperatures.

It is another object to provide alloy steel wires having a low relaxation or low creep property at room and elevated temperatures.

The relaxation property of steel wires used in prestressed concrete is of extremely important significance thereto, and an improvement in relaxation property can be made by the following prior method:

Music spring steel wires are subjected to tension, equivalent to the several ten percent of their tensile strength, at a temperature of from 150 to 500 C. to fix the resulting dislocation thereby lessening their relaxation loss.

It is very economically disadvantageous to subject music spring steel wires to high temperatures and high tensions in view of the cost of equipments and operation necessary to do so. The steel wire made of music spring steel is remarkably inferior in relaxation property at temperatures above room temperature, and it is therefore impossible to use as a high-temperature high strength material. And, in a structure made of prestressed concrete, the cost of steel wires used in the concrete constitutes a considerable portion of that of the structure, and therefore it is practically 4 difficult to adopt a special step or a high-alloy steel as a starting material. from the economic point of view.

The essence of the process of the present invention is that the process makes it possible for a low-alloy steel wire material to be improved in high-temperature relaxation property at a temperature ranging from room temperature to C.

The presence of carbon in a low-alloy steel wire mate-' rial to be treated according to this invention is indispensable for increasing the strengths of a wire to be obtained from the wire material. The wire will not have enough strength to be able to be used as such for prestressed concrete if prepared from the wire material containing less than 0.5 of carbon, and it will lower in workability and toughness if prepared from the wire material containing more than 0.95% of carbon.

The manganese and silicon present in ingots from which the wire material is made are necessary elements, as deoxidizers, to ensure the soundness of the final wire to be prepared from the wire material, and they are effective in obtaining the final wire having improved strengths and toughness. They will, however, have an adverse effect such that the final wire decreases in toughness and the like, if their respective contents are too high. Thus, they should be present in the wire material in such respective limited amounts of 0.3-1.0% and 06-22%.

In a steel such as the starting wire material, silicon is a substitution type atom for iron and the difference in length of atomic radius between the two will cause lattice distortion, thereby strengthening the ferrite. Thus, the dislocation occurring due to the latice distortion will from Cottrell atmosphere thereby interlocking nitrogen and carbon, and, particularly, the interlocking of the dislocation will be promoted by artificial aging. In addition, the presence of the silicon in the wire material will hamper the diffusion of the nitrogen and carbon in the material whereby the temperature at which the strain aging is effected, is shifted to the higher-temperature side than that for the ordinary steel. This is one of the characteristics of a silicon-containing steel. Therefore, the silicon content below a certain'level is hardly expected to have such effects as mentioned above. More particularly, the silicon will be little effective in lowering relaxation value if its content is less than 0.6%, while it will be more effective as its contents increase to 2.2% and will not furthermore be effective if its content exceeds this value. Its content should thus be in the range of from 0.6% to 2.2%.

Similar to the silicon in the wire material, the chromium therein is a substitution type element which can be expected to have the same effects as the silicon, and it should be present in the wire material in amount of 0.2- 1.2% to be effective in ensuring or enhancing the stabilization of the cementite, hardenability, mass effect and temper resistance of the material.

The addition to the wire material of at least one of alloying elements, such as titanium, niobium, vanadium and the like, each in an amount of 0.5% or less is conducive to the stabilizing of the carbide in the material so added, and the addition of the titanium and vanadium is intended to obtain the effects of deoxidizing and nitrogen-fixing in the added material. These three elements should be added to the material each in an amount of 0.5% or less, since they will deteriorate the cold workability of the material and will cost unduly much due to being expensive metals if added in amounts of more than 0.5%.

FIG. 1 contains plots of the results of a test on various steel wires for their relaxation value versus temperature of exposure;

FIG. 2 shows the relationship between the yield ratio obtained from a tension test and stress-relieving temperature;

FIG. 3 shows the relationship between the relaxation value and stress-relieving temperature;

wires in tensile properties with temperature of exposure;

FIG. 5 shows the characteristic relationship between the tensile properties of a silicon steel wire of this invention and stress-relieving temperature, as compared with that of an ordinary carbon steel wire;

FIGS. 6A, 6B and 6C are relaxation curves for the ordinary carbon steel wire and the silicon and chromiumsilic'on steel wires of this invention, respectively, the

temperature range, and, in this case, the silicon and chromium added have a remarkable effect on the relaxation of Samples 2 and 3. I

From the results obtained with Samples 2 and 3 which are low-allow steel wires according to this invention, the amounts of silicon and chromium to be contained in a wire according to this invention have been determined to be 0.6-2.2% (preferably, 0.96-2.2%) and 02-12%, respectively.

TABLE 1A Reduction of area Mechanical properties of drawn wire material Chemical composition, reached at percent the end of Diameter Reducdrawings, of wire an,kg./ zm.2.kg./ Elong, tion area, percent (mm. mm. nun. percent percent TABLE 1B Mechanical properties after stress relieving heat treatment Stress relieving heat treatment Reduction of Relaxa Temp. Time n.2, Elong., area, tion, Sample 0.) (min) kgJmm 2 kgJmm. percent percent percent 1 Hot stretched at 250 0.

curves showing the relaxation value versus temperature of exposure at the indicated temperatures;

. FIG. 7 shows the relaxation value for the three different .wires versus initial load applied thereto at room temperature; and

FIG. 8 shows the load loss for the test wires when subjected to the relaxation test in a corrosive solution.

The invention, particularly, the effects thereof will be illustrated by the following experiments.

Tables 1A and 1B summarize the chemical compositions, degrees of working by drawing (expressed by reductions of area in percent reached at the end of drawing), mechanical properties before and after stress-relieving heat treatment, and relaxation value at C. of five different wire samples, thereby showing the effects of the alloying elements present in the starting wire samples on the mechanical properties and on the relaxation values at room temperature of the final wire samples.

Samples 1 to 5 are a 0.47%-silicon steel wire, 0.96%- silicon steel wire, silicon-chromium steel wire, music spring steel wire, and hot-stretched steel wire prepared by subjecting a music spring steel wire to tension a 0.4 at a temperature of 250 C., respectively. And all the steel wires are 5 mm. in diameter.

FIG. 1 showsthe' relationships between the temperature of exposure and the relaxation value 100 hours after the start of the test, the relationships being respectively obtained when exposing the samples to temperatures in the range of 20'-200 C.

As seen from FIG. 1, the aptitude of relaxation of Sample 1 is approximately the same as that of Sample 4 which is a conventional steel wire for use in prestressed concrete, and his little influenced by the addition of silicon. Sample 5 show that it loses, at temperatures above 140 C., the eifect it previously obtained by hot stretching treatment in spite of its very low relaxation value at room temperatures.

Samples 2 and 3 have a much lower relaxation value than Sample 4, although the former change in relaxation value in the samemanner as the latter over the range of temperatures of exposure, that is, both of them are identicalxin aptitude of relaxationwith each other over said FIG. 2 shows the relationships between yield ratio (o' /0' X and stress relieving temperature, the relationships being respectively obtained with a carbon steel wire of conventional type and a silicon and a chromiumsilicon steel wire after the heat treatment in an air bath for 5 minutes.

As is clear from this figure, both the silicon and chromium-silicon steel wires greatly increase in (r /0' at the temperatures above 250 C. as compared with the carbon steel wire, and, particularly, the chromium-silicon steel wire shows its maximal yield ratio at a temperature of 330 C. and maintains this level at temperatures from 330 C. to 420 C. and even the silicon steel wire having its maximal yield ratio at 360 C. is remarkably superior to the conventional carbon steel wire in stability of yield ratio at temperatures above the 360 C.

FIG. 3 show the relationships between stress relieving temperature and relaxation value obtained 24 hours after the stress relieving treatment. As is apparent from this figure, the carbon steel wire gradually decreases in relaxation value while the silicon and chromium-silicon steel wires sharply decrease in relaxation value at approximately 200 C. continue the decreasing as the temperature rises, reach the lowest value at 400450 C. and then tend to increase at temperature above these ones.

In view of the mechanical properties and relaxation values which the final wires of this invention should preferably have as those for use in prestressed concrete, the stress relieving temperature is determined by the inventors to be in the range of 250-550 C.

This invention will be better understood by the following examples.

EXAMPLES Samples of silicon steel wire material and those of chromium-silicon steel wire material were respectively heat treated. In addition, samples of carbon steel wire material for conventional use in prestressed concrete were also heat treated for comparison. Table 2 shows the chemical compositions of the samples, and Table 3 indicates the conditions under which they were heat treated.

As previously described, there are diiferences in type of heat treatment between the wire material which can be cooled at a cooling rate higher than its lower critical cooling'rate in the cooling. step, and that which cannot ical properties of the thus-treated samples, that is, final A wire products are shown inTable' 5.

TABLE 4 Reduction Soaking conditions Wire dia.,obof area 013- for relieving stress tained after tained after drawing drawing, Temp. Time Sample (mm.) percent C.) (min).

Carbon steel wire material 5 72. 3 360 2.6 Silicon steel wire material 2. 9 72. 3 420 2. 5 Chromium-silicon steel wire material 5 76. 8 420 2. 5 Silicon steel wire materi 7 66. 420 2. Chromium-silicon steel wire material 7 64. 6 420 2. 5

TABLE 5 p 0.2% 0.05% Yield Tensile offset ofiset Modulus ratio, strength, stress, stress, of elas vM/a-B Sample an em v0.05 ticity, Elong., X100 symbol Sample kgJmmfi kg./mm. kgJmmJ kgJmm 2 percent percent (A) Carbon steel wire product 183. 2 160. 3 150. 4 21). 9x10 6. 5 I 91. 9 (B). Silicon steel wire product 177. 2 165. 3 153. 3 217x 7. 0 93. (C). Chromium-silicon steel wire product 178. 2 173. 3 170. 1 203x10 7. 0 97. (D) Silicon steel wire product 176. 0 159. 0 151. 0 20. 8X10 7. 0 90. (E) Chromium-silicon steel wire product 165. 0 158. 0 149. 0 20. 9X 10 7. 0 95;

be because of its greater size of diameter (or thickness) and its composition. 'It may therefore be necessary to determine which type of heat treatment the wire material should be subjected to, by making a preliminary heat treatment test using its test samples prior to subjecting it to the heat treatment.

There are hereunder described the effects of exposure temperature on the mechanical properties, including re laxation property, of the representative wire products ((B) and (C)) in comparison with those of the conventional wire product (A).

Table 6 shows the results obtained from the tensionv tests on the wire products, made at room temperature TAB LE 2 Chemical compositions, percent Sample symbol Sample C Si M11 P S Cu Cr (A) Carbon steel wire material 0. 73 0. 25 0. 86 0. 018 0. 011 0. 05 0. 053 (B) Silicon steel wire material 0. 72 1. 22 0. 52 0. 010 0. 005 0. 05 0. 030 (C) Chromium-silicon steel wire material 0. 54 1. 0. 74 0. 013 0. 009 0. 04 0. (D) Silicon steel wire material 0. 76 1. 49 0. 57 0. 012 0. 005 (E) Chromium-silicon wire material 0. 54 1. 45 0. 74 0 013 0. 009 04 0. 60

TABLE 3 Heating Reheating conditions Coohng conditions conditions Diam- Sample eter Temp. T me Temp. Temp. Time symbol Sample (mm.) 0.) (mm) Method of cooling 0.) C.) (min.)

(A) Carbon steel wire material *9. 5 940 2. 0 Dipping in lead bath (B) Silicon steel wire material 5. 5 880 2.0 Air-cooling (O) -i Chromium-silicon steel wire material 9. 0 840 2. 2 .do (D) Silicon steel wire material 12. 0 940 2. 5 Dipping in lead bath (E) Chromium-silicon steel wire material 10. 4 840 2. 2 -do *The carbon steel wire material was, in the cooling step, subjected to the usual lead patenting at 480 C. for 2.6 minutes.

The samples so heat treated were then cold drawn and further heat treated to relieve it of its stress simultaneou'sly with eifecting the artificial aging of it, under the (20 C.), 50 C., C.,' C., 200 C., 250 C., 300 C., 350 C. and 400 C.

FIGS. 4A, 4B and 40 contain plots of the test results on tensile strength, 0.2% offsetstress and 0.05% offset operational conditions described in Table 4. The mechan- 7 stress versus temperature of exposure at the indicated temperatures, respectively, the test results being shown inTableG FIG. shows the comparison of the stress-relieving temperature versus mechanical properties so obtained be- TAB LE 6 0.2% 0.05% ofiset 4 ofiset Modulus of g p Temp. 01' Tensile stress, stress, elasticity, Elong.,

exposure, strength, 60.2, 00.05, X 100 mm., 60.2/05. 60.05/03, Wire tested p C. o kgJmm. kg./m.m. kgJmm. I kgJmm 2 percent percent percent Carbon steel wire (A) 183. 2 168. 3 150. 4 20. 9 6. 5 91. 9 82. 2

' 50 178. 2 158. 3 148. 3 20. 7 6. 5 88. 8 83. 1 100 174. 2 148. 3 136. 4 20. 7 6. 5 85. 1 78. 3 I 150 174. 2 145. 4 130. 4 21. 4 6. 5 83. 4 74. 8 200 168. 3 133. 4 118. 5 20. 7 7. 0 79. 3 70. 4 250 166. 2 130. 4 100. 6 19. 7 11. 5 78. 4 60. 5 300 147. 3 118. 5 97. 1 19. 7 12. 0 79. 4 65. 0 350 133. 4 101. 6 74. 7 18. 6 13. 0 76. 1 56. 0 l 400 113. 5 89. 6 68. 7 17. 9 13. 0 79. O 60, 5 51110011 steel wire (B) 177. 2 165. 3 153. 3 20. 7 7. 0 93. 2 86. 5 50 177. 2 165. 3 153. 3 20. 4 7. 0 93. 2 86. 5 100 176. 2 153. 3 143. 4 l9. 8 7. 0 87. 0 87. 1 150 171. 2 149. 3 135. 4 19. 7 7. 0 87. 2 79. 1 200 171. 2 143. 4 126. 4 20. 0 8. 0 83. 7 73. 8 250 170. 2 129. 4 99. 5 20. l 11. 0 76.0 58. 5 300 163. 3 119. 5 90. 6 19. 6 13. 0 73. 2 55. 5 v 350 144. 3 112. 5 88. 6 20. 3 13. 0 77. 9 61. 4 400 127. 5 95. 2 73. 7 20. 1 13. 0 75. 8 57. 6 Chromium-silicon steel wire (0) 178. 2 173. 3 17 2 3 7. 0 97. 2 95. 5 50 178. 2 173. 3 170. 2 20. 3 7. 0 97. 2 95. 5 .100 174. 2 169. 3 162. 3 20. 0 7. 0 97. 1 93. 1 159 172. 3 158. 3 147. 4 19. 4 7. 0 91. 9 85. 5 200 170. 2 150. 4 138. 4 19. 3 8. 0 88. 3 81. 3 250 169. 3 142. 4 129. 4 19. 0 12. 0 84. 1 76. 5 k 300 165. 3 141. 4 125. 4 18. 3 14. 0 85. 6 75. 9 350 153. 3 128. 5 105. 6 17. 4 14. 0 83. 8 68. 9 400 132. 4 114. 5 93. 6 16. 7 14. 0 86. 5 70. 7

' Room temperature.

As is seen from FIG. 4, the silicon steel wire and chromium-silicon steel wire are generally superior to the carbon steel wire in tensile strength, and the former maintaintheir tensile strength at a certain, substantially constant level when kept at temperatures up 0 300 C. above which the strength decreases, whereas the latter linearly decreases in tensile strength at temperature above 150 C. The said three steel'w ires are identical with our another in the manner that their 0.2% and 0.05% offset stresses decrease as the temperature of exposure rises, and, however, the ratio of each of the stresses at the indicated temperatures to the corresponding tensile strength at room temperatures is higher for the silicon and chromiumsilicou steel wires than for the carbon steel wire.

Table 7 shows how the silicon and chromium-silicon steel wires according to this invention decrease in mechanical properties at elevated temperatures up to 400 C.. This was foundby heating the test samples of these wires to'the indicated temperatures in 10 minutes, maintaining them at these temperatures for 5 minutes and then measuring the stress versus the strain in the next 5 minutes. Y

.The temperature at which the carbon steel wire decreases in tensile strength to 90% of its original value is 250 C., while those for the silicon and chromiumsilicon steel wires are 310 C. and 325 C., respectively. Thecarbon steel wire also decreases in 0.2% offset stress to 90% of its original value even at a temperature of C., while the silicon and chromium-silicon steel wires decrease to the same percent at temperatures 80 C. and 140 C., respectively. These facts indicate that the lowalloy steel wires according to this invention have high heat resistance as compared with the carbon steel Wllfi.

. .ITABLE 7 Temp. at which Temp. at which rupture occurs toyield occurs to- Stress applied, T -C-, Si- C- Siex ressedin ercent. steel steel Cr-Si steel steel (Jr-Si ofgtrength' p (A) (B) I (0) (A) (B) (0) tween the post-drawing silicon steel wire material and carbon steel wire material each described in Table 3.

As is obvious from FIG. 5, the carbon steel wire A begins to increase in both 0' and a at approximately 200 450 C. Particularly, the value of 0 /11 that is yield ratio for the wire A is 95%, while that for the wire B is as high as 98%. This clearly indicates that the wire B is extremely superior to the wire A in mechanical properties at the elevated temperatures.

Table 8 shows the relaxation values for the carbon, silicon and chromium-silicon steel wires, the values being obtained from the test in which were applied to the initial steel wires the initial loads equal to 70% of their tensile strengths at the seven diflerent exposure temperatures in the range of 20 to 200 C., respectively. These values are the ones which were measured 100 and 1000 hours after the start of the test.

FIGS. 6A, 6B and 6C contain plots of the results of said test on the carbon, silicon and chromium-silicon steel wires for relaxation values versus times of exposure under the initial loads equal to 70% of the (ultimate) tensile strengths of the wires and at the indicated temperatures, respectively.

TABLE 8 Relaxation value, percent (A) C-steel (B) Si-steel (C) Cr-Si steel wire at wire atwire at- Iemp. of 100 1,000 100 1,000 100 1, 000 exposure, 0. hr. hr. hr. hr. hr. hr.

Room temp 3.0 4. l 1. 0 2.0 0. 8 1. 2 50 7. 7 12.2 2. 8 4. 8 1 3. 5

Wires prepared according to this invention have a smaller relaxation value than the carbon steel wire. And the ;relaxation value for the carbon steel wireat an exposure temperature ,of 200. C., ,is,20.0%,. vvhile those for ,the..

silicon and chromium-silicon steel wires are respectively 17.5% and 13.5%, this showing that the low-alloy steel wires prepared according to this invention have superior relaxation property to thatof the carbon steel wire.

FIG. 7 shows thetest resultsobtained from the testin which various initial loads were appliedto the to-be-.

tested steel wires at C. for'24 hours tdsee, the effect of the loads on the relaxation values for the wires at the time 24 hours after the start of the test- According to FIG. 7, the silicon and chromium-silicon steel wires show their respective relaxation values which are approximately the same when the initial loads equivalent to 67.5% of their tensile strengths are respectively applied to the wires, and the values for the silicon steel wire are higher as the loads (expressed in percent of the tensile strength of the wire) are heavier, the values being likely to increase in proportion to the increase of the loads.

The relaxation values for the carbon steel wire are much greater than those for the silicon and chromiumsilicon steel wires, and the increase of the values for the former is also likely to be in proportion to that of the loads.

When, for instance, the initial loads are respectively equivalent to 70% of their tensile strengths, the carbon, silicon and chromium-silicon steel wires have their respective relaxation values of 2.7%, 1.0% and 0.8% after the start of the test. The values for the latter two are only 32.8% and 29.5% of that for the former. This indicates that the relaxation values for the low-alloy steel wires of this invention will be much lower than that for the carbon steel wire unless the initial loads applied are impractically or extraordinarily heavy.

FIG. 8 contains plots of test results on the relaxation value versus loading time, obtained from the test in which the diiferent test wires as previously mentioned were dipped in a 2-N aqueous solution of ammonium nitrate, as a corrosive solution, at a temperature of 50 C. while applying to the wires the initial loads equal to 70% of the tensile strengths thereof, respectively.

As is seen from FIG. 8, the silicon and chromiumsilicon steel wires, even if they are low-alloy ones, do not cause their rupture due to stress corrosion and are superior in relaxation loss to the carbon steel wire when they are subjected to the relaxation test even in the corrosive solution.

What we claim is:

1. A process for manufacturing a low-alloy steel wire having low relaxation property at room and elevated temperatures, characterized by heating a starting lowalloy steel wire material comprising, by weight, 0.50- 0.95% of carbon, 03-10% of manganese, 06-22% of silicon as the additive, and balance iron and incidental impurities, to a temperature above the A transformation point of the wire material, thereafter cooling the heated wire material to a temperature just above the Ms point thereof when the material is such that its cooling rate in this cooling step is not lower than its lowercritical cooling rate, and then reheating the cooled wire material, while continuously passing the Wire material through the heat treating devices, to obtain a wire material with a highly cold-workable sorbitic structure and high tensile strength, the thus-obtained wire material being subsequently draw worked and subjected to artificial aging to obtain the desired low-alloy steel wire.

2. A process for manufacturing a low-alloy steel wire having low relaxation property at room and elevated tem-' peratures, characterized by heating a starting low-alloy steel wire material comprising, by weight, 0.500.95% of carbon, 0.31.0% of manganese, 06-22% of silicon as the additive, at least one selected from the group consisting of 0.21.2% of chromium, up to 0.5% of moylbdenum, up to 0.5 of titanium, up to 0.5% ,of niobium and up to 0.5 of vanadium, as the second additive, and balance iron and incidental impurities, to a temperature above the A transformation point of the wire material,

thereafter cooling the heatedwire material to a temperature just above the Ms point thereof when the material is such that its cooling rate in this cooling step is not lower than its lower critical cooling rate, and then reheating the cooled wire material, while continuously passing the wire material through the heat treating devices; to obtain a wire material having a highly cold-workable sorbitic structure and high tensile strength, the thus obtained material being subsequently draw worked and subjected to artificial aging to obtain the desired lowalloy steel wire.

3. A process for manufacturing a low-alloy steel wire having low relaxation property at room and elevated temperatures, characterized by (1) heating a starting lowalloy steel wire material comprising, by weight, 0.50- 0.95% of carbon, (LS-1.0% of manganese, 0.62.2% of silicon as the additive, and balance iron and incidental impurities, to a temperature above the A, transformation point of the wire material, ,(2) thereafter cooling the heated wire material by being dipped in a bath of a member selected from molten lead and salt at a temperature of 450-650 C. for at least 20 seconds and withdrawn therefrom to be cooled to room temperature, when the material is such that its air-cooling rate is lower than its lower critical cooling rate because of its large size in diameter or thickness,,to obtain a wire material having a highly cold-workable sorbitic structure and high tensile strength, said two steps (1) and (2) being continuously carried out, (3) draw-working the thus-obtained wire material and then (4) subjecting the draw-worked wire material to artificial aging to obtain the desired low-alloy steel wire.

4. A process for manufacturing a low-alloy steel wire having low relaxation property at room and elevated temperatures, characterized by (l) heating a starting lowalloy steel wire material comprising, by weight, 0.50- 0.95% of carbon, 0.3-1.0% of manganese, 06-22% of silicon as the additive, at least one selected from the group consisting of 0.2-1.2% of chromium, up to 0.5% of molybdenum, up to 0.5% of titanium, up to 0.5% of niobium and up to 0.5% of vanadium, as the second additive, and balance iron and incidental impurities, to a temperature above the A transformation point of the wire material, (2) thereafter cooling the heated wire material by being dipped in a bath of molten lead at a temperature of 450-650" C. for at least 50 seconds and withdrawn therefrom to be cooled to ambient temperature, when the material is such that its air-cooling rate is lower than its lower critical cooling rate because of its large size in diameter or thickness, to obtain a wire material having a highly cold-workable sorbitic structure and high tensile strength, ,(3) draw-working the thus-obtained wire material and then (4) subjecting the draw-worked wire material to artificial aging to obtain the desired low-alloy steel wire.

5. A process according to claim 3 wherein the cooling 2) is achieved by air-cooling the wire material to ambient temperature at a cooling rate of not higher than 3 C./

sec.

6. A process according to claim 4 wherein the cooling (2) is effected by air-cooling the wire material to ambient temperature at a cooling rate of not higher than 1.5. C./

sec.

7. A process according to claim 1 wherein the wire material is less than 6 mm. in diameter or thickness.

'8. A process according to claim 2 wherein the wire material is less than 9.5 mm. in'diameter or thickness.

3 C., that just above the Ms point is 310-440" C. and that to which the material is reheated is 450-650 C.

12. A process according to claim 2, wherein the temperature above the A transformation point is 820950 C., that just above the Ms point is 310-440 C. and that to which the material is reheated is 450-650 C.

13. A process according to claim 3, wherein the temperature above the A transformation point is 820- 950C.

14. A process according to claim 4, wherein the temperature above the A transformation point is 820- 950" C.

15. A process according to claim 1, wherein the artificial aging is efliected at a temperature of 250-550 C.

14 16. A process according to claim 2, wherein the artificial aging is effected at a temperature of 250-550 C.

References Cited UNITED STATES PATENTS 3,458,365 7/1969 Nickola et a1. 14812.4 3,506,468 4/1970 Geipel et a1. l48--12.4 3,532,560 10/1970 Tomioka et al. 14812.4

10 L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner US. Cl. X.R. 

